A receiver for a radio frequency signal is configurable to operate in various applications. For example, the receiver can be used in TV receivers or in receivers used in a radio communications system. The receivers in such radio communications systems are usually included in wireless phones and in transceiver stations. An exemplary radio communications system is a cellular system that is in accordance with a particular specification, such as "Global System for Mobile Communications" (GSM), "Advanced Mobile Phone System" (AMPS) or "Code Division Multiple Access" (CDMA).
An example of an RF receiver is a direct-conversion receiver in which the local signal has a frequency that is set to be the same as the frequency of the RF signal. In such a direct-conversion receiver, the intermediate frequency is zero and the mixer transforms the RF signal into the baseband.
The transformed signal in the baseband has typically a low power level and requires subsequent amplification. Thus, the direct-conversion receiver has a high gain for the signal in the baseband. Because of this high gain, any component of the baseband that is slightly offset from the frequency zero becomes critical for a satisfying operation of the direct-conversion receiver. Any component of the baseband which is offset at a frequency zero is subsequently generally referred to as "DC offset." For instance, the DC offset may cause the direct-conversion receiver to become overloaded because the DC offset is also subject to the high gain and a final amplifier is driven into saturation. The DC offset may be caused by a mismatch of internal receiver components or frequency components that mix and produce products that fall into the baseband.
A direct-conversion receiver in a radio communication system is configured to have a varying gain in order to track the varying signal strength of the received RF signal. Because of this varying gain, the magnitude of the DC offset at the output of the mixer is constantly changing. Moreover, the varying gain complicates conventional methods to compensate for or cancel the DC offset. The faster the DC offset can be compensated or canceled, the sooner a cellular phone can receive good data. Particularly for radio communications systems that operate in time division duplex (TDD), the faster the DC offset can be corrected, the higher the TDD transmission rate. A high TDD transmission rate is desired, for example, because a high TDD rate improves voice quality in cellular phones by reducing the delay or echo that is typically canceled.